As a consequence of the progress made in integrated circuit technology, the spacing between the metal lines on any given plane of an integrated circuit has become less and less, now extending into the submicrometer range. By reducing the spacing between conductive members in the integrated circuit, an increase in capacitive coupling occurs. This increase in capacitive coupling causes greater crosstalk, higher capacitive losses and increased resistor capacitor (RC) time constant.
Inorganic materials such as silicon dioxide and silicon nitride have been traditionally used in the microelectronics industry as insulating and passivating materials in the manufacture of integrated circuits. However, as the demand for smaller, faster, and more powerful devices becomes prevalent new materials will be needed to enhance the performance and the efficient manufacture of these devices.
To meet these enhanced performance and manufacturing criteria, considerable interest in high performance polymers characterized by low dielectric constant, low moisture uptake, good substrate adhesion, chemical resistance, high glass transition temperatures (e.g., T.sub.g &gt;250.degree. C.), toughness, high thermal and thermal-oxidative stabilities, as well as good optical properties are increasingly gaining momentum. Such polymers are useful as dielectric coatings and films in the construction and manufacture of multichip modules (MCMs) and in integrated circuits (IC), in electronic packaging, in flexible film substrates, and in optical applications such as in flat panel displays and the like.
In order to reduce capacitive coupling, much effort has been directed toward developing low dielectric constant (low-K) materials to replace conventional dielectric materials that are interposed between the metal lines on a given layer and between layers. Many conventional electronic insulators have dielectric constants (.epsilon.) in the 3.5 to 4.2 range. For example, silicon dioxide has a dielectric constant of 4.2 and polyimides typically have dielectric constants from 2.9 to 3.5. Alternatively, the silicon dioxide can be decreased by adding fluorine in place of oxygen to yield a substance with a dielectric constant of approximately 3.5. Some advanced polymers have dielectric constants in the 2.5 to 3.0 range. Materials in the 1.8 to 2.5 range are also known, but such materials have had associated therewith severe processing, cost and materials problems.
The lowest possible, or ideal, dielectric constant is 1.0, which is the dielectric constant of a vacuum. Air is almost as good with a dielectric constant of 1.001. With this recognition of the low dielectric constant of air, attempts have been made to fabricate semiconductor devices using porous materials as an insulator. Thus, by incorporating air, the dielectric constant of a substance can be lowered.
For example, porosity can be added to silicon dioxide to decrease its effective dielectric constant. Thus, if 50 percent of the volume of a dielectric is air, the effective dielectric constant of the porous silicon dioxide can be calculated by multiplying the percentage of the total volume of the porous dielectric that is air (i.e., 50 percent) times the dielectric of air (1.001 or for ease of calculation 1) and add to it the percentage of the total volume of the porous dielectric that is, for example, silicon dioxide (.epsilon.=4). Thus, for s 50/50 mix of silicon dioxide and air the dielectric constant of the porous material is as follows: .epsilon.=0.5*4+0.5*1=2.5. Porous materials, such as the one described above, can be made up with as high as 90 percent air. However, such porous materials suffer from a number of drawbacks, such as, for example, a lack of mechanical and reliability attributes.
Another solution to lowering the dielectric constant of silicon dioxide is to use a spin-on-glass (SOG), which is generally a siloxane based material of low molecular weight, to lower the effective dielectric constant of silicon dioxide. The SOG is heat treated after deposition thereby completing a network of chemical bonds. This creates a "cage structure" of SOG and makes the density of the SOG less than that of silicon dioxide. As a result the dielectric constant of the SOG is lower than that of just silicon dioxide. However, such a reduction in the dielectric constant of a substance can be insufficient for some newer electrical applications, for example, high speed integrated circuits.